Stability of partial difference equations governing control gains in infinite-dimensional backstepping

We examine the stability properties of a class of LTV di"erence equations on an in&nite-dimensional state space that arise in backstepping designs for parabolic PDEs. The nominal system matrix of the di"erence equation has a special structure: all of its powers have entries that are −1, 0, or 1, and all of the eigenvalues of the matrix are on the unit circle. The di"erence equation is driven by initial conditions, additive forcing, and a system matrix perturbation, all of which depend on problem data (for example, viscosity and reactivity in the case of a reaction–di"usion equation), and all of which go to zero as the discretization step in the backstepping design goes to zero. All of these observations, combined with the fact that the equation evolves only in a number of steps equal to the dimension of its state space, combined with the discrete Gronwall inequality, establish that the di"erence equation has bounded solutions. This, in turn, guarantees the existence of a state-feedback gain kernel in the backstepping control law. With this approach we greatly expand, relative to our previous results, the class of parabolic PDEs to which backstepping is applicable. c © 2003 Published by Elsevier B.V.